The method presently in use for attenuating noise generated by the commercial jet aircraft engines was initially developed in the late 1960's and early 1970's by the manufacturers of such aircraft. This technology consists of an acoustic lining system for the engine, having a sandwich-type construction consisting of sintered metal mesh facing the engine air flow paths in the inlet and fan exit ducts. This mesh is bonded to the duct liner skin with a precisely sized and spaced hole pattern tuned to the primary noise frequency generated in that portion of the engine at the critical operating mode being silenced. The metal mesh and perforated duct liner skin are bonded to a honeycomb structure backed with a solid skin.
At the time this system was developed, the industry objective was to meet the requirements of Federal Air Regulation Part 36, Stage 2. This system remains the primary industry development for attenuating the noise generated by narrow-bodied jets, as it has been perceived as the only system adequate for service in its unique operating environment in the jet engine. For example, JT3D commercial jet engines installed on 707 and DC8 aircraft have used and continue to use this system.
Sound attenuation with this system is accomplished by the Helmholz Resonator effect whereby cavities in the honeycomb dissipate acoustical energy after its admittance through the metal mesh and perforated skin which has been placed between the honeycomb and the sound generating elements of the engine. The solid skin backing in the honeycomb is impervious to acoustical energy radiation and prevents acoustical transmission. Some structure-borne sound transmission is transmitted by the sandwich construction, but this is of a secondary nature. Loss in engine performance, however, has been associated with air leakage through the honeycomb lining.
The noise generated by the fan section of jet engines occurs at discrete primary frequencies which vary depending on engine model, fan speed, and location along the duct. Attenuation of such noise using the above mentioned method has the potential to achieve the initial goal of and compliance with Stage 2 of the Federal Air Regulations, Part 36. Therefore, industry research has concentrated on the precise tuning of the lining design to the engine noise source characteristics, with emphasis on acoustic parameters of different dimensional and material properties of the porous metal facing sheet, honeycomb core, and the solid backing sheet. Results have been barely adequate with differing degrees of economic and operational penalties.
The system described above is estimated to be capable of producing an attenuation of 6 to 11 DB and requires the meticulous fine tuning of critical parameters such as skin hole size, metal mesh grid and thickness, honeycomb material makeup, and cavity dimension and thickness. This system is designed to attenuate only one primary frequency for a given combination of critical parameters. The result has been marginal compliance with the Stage 2 requirements of Federal Air Regulations, Part 36. Moreover, Stage 3 requirements have not yet been reached with the present system without resort to measures which result in a high cost and high risk solution to the noise problem and a significant engine performance penalty, exposure to catastrophic engine failure, and continuing maintenance problems. A further problem with such measures would involve obtaining airframe and engine manufacturer approval for the attendant inlet airflow changes.